Hide the case and air parts (if they are showing).
Select the filter.
Open the Materials quick edit dialog in ONE of the following ways:
Material DB Name = My Materials
Type = Resistance
Name = Baffle
Flow Direction: Click the ... button
Click the Z button on the Flow Direction dialog.
(On the Material quick edit dialog) Normal Direction 1 = Global X.
Normal Direction 2 = Global Y (automatically selected).
Click Apply.